Electron emission device of high current density and high operational frequency

ABSTRACT

An electric device operable with a THz-range frequency of the device output is presented. The device comprises a photocathode installed in either one of a diode, triode and tetrode configuration, and is exposed to illumination. In some embodiments of the invention, the device is configured as a diode and photomixing is used for illumination of the photocathode with light in the THz range, the diode converting this input light signal into an electrical output in the THz range, which operates a signal transmitter/receiver. In some other embodiments of the invention, the device is configured as a triode or tetrode, where the electrodes have small dimensions (about 1 micron or less) and are spaced from one another a distance not exceeding 1 micron. The photocathode is kept under certain illumination, and electrical signal applied to one of the electrodes results in the THz output at one of the other electrodes.

FIELD OF THE INVENTION

This invention relates to an electric photoemission device capable ofoperating at high (e.g. Terahertz) frequencies and with highphotocurrent density.

BACKGROUND OF THE INVENTION

Applications requiring electric devices operable at Terahertz (THz)frequencies are desirable in many areas such as medical imaging,security detection of hostile objects and noxious chemicals, and others.The THz regime, commonly defined as the range from 300 GHz to 10 THz,corresponds to wavelengths between 1 mm and 30 μm. THz radiation is ableto penetrate a variety of materials which are opaque to visible light,such as clothing or paper. On the other hand, THz radiation is absorbedby water and organic substances, materials commonly perceived astransparent. These unique absorption properties lend themselves forvarious screening and imaging techniques.

A current approach for operating at THz frequencies is based on the useof CMOS technology, more specifically the use of GaAs semiconductorantenna. However, for efficient THz generation in such antenna,employing GaAs photomixer, the laser wavelength has to be smaller thanthe semiconductor bandgap and the laser sources must be powerful, and asmany THz absorption lines are tens or hundreds of GHz broad, must have alarge mode-hop-free tuning range. The use of GaAs semiconductor antennais problematic due to the limit on the speed of electrons in asemiconductor. Therefore the THz range is a spectral gap, in whicheffective operation of electronics based complex systems are hard toimplement.

GENERAL DESCRIPTION OF THE INVENTION

There is a need in the art in an electric device capable of operatingwith both high photocurrent density and high frequencies.

The inventors have found that appropriate arrangement of a photoemissionbased electric device, in which electrons travel in a free space(vacuum), provides for obtaining desirably higher operationalfrequencies and emission current densities.

The limitation on current density appears to be a result mainly ofphotocathode degradation and of space charge. The causes of photocathodefatigue and instability are usually divided into two categories: thosewhich are relevant or dominant in the absence of current flow, and thosewhich are relevant or dominant in the presence of current flow.

Three principal factors fall in the first category: oxidation of thephotocathode by residual gases, excess or shortage of Cs coating, andoverheating. These effects are essentially consequences of variousfailures during the photocathode production and/or storage process.Oxidation, for example, depends on the presence of residual gases, whichis a result of either insufficient pumping/degassing of the substrateand other envelope parts or due to the formation of leaks along thevacuum seal. Excess or shortage of Cs is typically a result of lowactivation temperature or insufficient activation time, respectively.Overheating of the photocathode (maximal temperature is typically in therange 100° C.-200° C.) can occur during storage or by strong absorbedlight. Such effects can be avoided by taking necessary precautions andusing optimal production and storage processes.

The main causes for photocathode deterioration in the presence ofcurrent flow are of a more fundamental nature.

One such cause is known as ion bombardment. The gas pressure withinconventional phototubes is usually in the range of 10⁻⁶ to 10⁻⁸ torr.This pressure is high enough to allow for occasional collisions betweenemitted, accelerated photoelectrons and gas molecules, resulting in theproduction of positive ions. These ions are accelerated by the electricfield in the phototube and cause damage to the photocathode when theystrike its surface. Ion bombardment is the most significant fatiguemechanism in conventional phototubes, photomultipliers, etc.

The high resistance of semitransparent photocathodes also contributes totheir deterioration. The sheet resistance of semitransparentphotocathodes is very high, reaching 10⁶-10⁷ Ω/sq depending on the typeof photocathode. As a result, when current is emitted from asemitransparent photocathode, a relatively large potential differenceappears between the photocathode's inner area and the conductive ringthat serves as an electrical contact to the photocathode. This potentialdifference can, in turn, lead to the following mechanisms of performancedeterioration. First, the inner areas of the photocathode becomepositively charged. Consequentially, only the most energetic electrons(if any) are able to overcome the resultant positive potential and beemitted. This effect prevents high emission current, but does not causeirreversible damage to the photocathode structure. Second, asufficiently large potential difference between the inner photocathodeareas and the electric contact can cause electrolytic decomposition ofCs—Sb (or K—Sb, etc.) compound. This irreversible effect is easilyrecognizable by a change of the photocathode's color. Third, the flow ofsufficiently high current through an electrically resistive medium canlead to dissipation of large amounts of heat, and consequentially, tooverheating of the photocathode material.

The highest values of the photocurrent density under which transparentalkali-based photocathodes remain stable are usually quoted inprofessional literature as 1-10 μA/cm², apart from S-20 photocathodeswhich were observed to remain stable at current densities of severalmA/cm². The currents obtained from reflective mode photocathodes are notlimited by the high in-plane photocathode resistance and should beconsiderably higher. For example, an experiment carried out by theinventors proved the stability of CsSb reflective-mode photocathode atabout 1 mA/cm² and 25V for at least half a year.

In the case of the pulsed photocathode currents, the peak current valuesmay be much higher than for continuous wave operation. The averagecurrents (calculated while taking into account the duty cycle of thepulses) are still limited by the photocathode resistance issues. Forexample, pulse currents (pulse length of 1-10 μs) of more than 1 A (froma couple of cm²) were extracted from the commercially availablephototubes Φ-13 and Φ-22 with S-20 reflective-mode photocathode [A. G.Berkovsky, V. A. Gavanin, and I. N. Zajdel, Vacuum PhotoelectronicDevices, Moscow (1976)].

A current density of 1 A/cm² is close to the space charge limit for thephototubes of conventional dimensions. Indeed, according to the Child'slaw

${J = {2.33 \cdot 10^{- 6} \cdot \frac{V^{3/2}}{d^{2}}}},$

the limiting cathode-anode distance for obtaining J=1 A/cm² in the spacecharge regime can be expressed as

d=15·10⁻⁶ ·V ^(3/4).

For an anode voltage of 100 V this distance is about 0.5 mm. If thecathode and anode are further apart, then the anode current would belimited by space charge effects and not by the photocathode performance.Therefore, it is practically impossible to reach higher currentdensities with the conventional tubes.

The technique developed by the inventors provides a solution to theproblems mentioned above and allows for a high-frequency andhigh-density photocurrent device. Furthermore, the technique disclosedin the present application utilizes small device dimensions to overcomethe limitation on charge carrier velocity and thus present a deviceoperable at high (e.g. THz) frequencies.

The disclosed technique takes advantage of the earlier techniquesdeveloped by the inventor of the present application and disclosed inWO05008711 and WO06077595, according to which an electron emissiondevice includes a photocathode and anode electrodes and possibly also agate electrode, where the cathode is exposable to suitable illuminationto induce emission of electrons therefrom.

In a device according to the present disclosure, the cathode and/oranode electrode areas, as well as the inter-electrode distances, are ofmicron to sub-micron dimensions, preferably on the order of tens tohundreds of nanometers. The emitted electrons traverse theinter-electrode cavities, and their trajectories are determined by theelectrode potentials.

Free electrons can be induced to move very fast compared to latticeelectrons (or holes) in a semiconductor material. By making thedimensions of the device small, the average “transit time”, which is thetime it takes an electron to complete its trajectory within the device,can be made very short, and fundamentally better than in CMOS devices.Also, the smaller the device's dimensions, the higher its maximalfrequency of operation, as determined by the mutual conductance tocapacitance ratio. The operation of such a device, however, cruciallydepends on the amount of emitted photocurrent density.

As mentioned above, one of the possible limitations on current densityis the space charge limit; the limit on emission current density as aresult of the repelling force of previously emitted electrons within thecavity. In a device according to the present disclosure, this limit isrelatively high due to the very small inter-electrode distances.According to Child's Law, the maximal current (due to space charge) isinversely proportional to the cathode-anode distance squared. If thisdistance is made very small, then space charge limitation issignificantly lessened.

Another advantage of a device according to the present disclosure isthat it operates at relatively low voltages (i.e. potential differencesbetween the electrodes), e.g. of the order of a few volts (as opposed toeven thousands of volts used in some current applications). This isimportant also with regard to degradation of the cathode, and hence withregard to the emitted current density and the frequency of operation.

The problem of ion bombardment could be avoided in the device accordingto the present disclosure not only by the small cavity dimensions,which, along with suitable vacuum, imply a very small chance ofcollision, but also by a possibility of operating with small potentialdifferences between the electrodes and hence low energy of theparticles. The probability of collisions between emitted electrons andgas molecules is significantly reduced due to the short cathode-anodedistance, which is shorter than the mean free path of the electrons(which is about 4 mm at a pressure of 1 torr), and to the very smallvolume of the device envelope, in which the number of residual gasmolecules is extremely small. (In a volume of 3μ×3μ×0.5μ at a pressureof 10⁻⁵ torr the number of gas molecules is of the order of one).Moreover, if the cathode-anode potential difference is smaller than theionization energy of the residual gases, then ionization by acceleratedelectrons cannot take place. The ion bombardment effect can beeliminated by keeping the anode-cathode voltage of a value not exceedingthe ionization voltage (which is about a few tens of volts), e.g. usingthe anode-cathode voltage of about 15 Volts. It should be understoodthat the present device can operate with higher voltages (e.g. 100V) inorder to provide higher operational power, but in this case thephotocathode life time may be shorter.

Another electrode damaging process is the heating of the material as aresult of current flow due to an internal voltage drop. Furthermore,because the electrode area is small, heat dissipation is improved. Forexample, in a device according to the present disclosure, the cathodemay be surrounded and covered on the non-emitting surface by aconductive layer for improved heat dissipation.

According to the present disclosure, in order to avoid the effects ofhigh photocathode resistance, the photocathode can be grown on a highlyconductive (but optically transparent) layer, e.g. a thin Cr film. Theplacement of a heat conducting grid over a large portion (preferablyall) of the photocathode area and at a sub-micron distance from thephotocathode serves to keep the photocathode from overheating by meansof improved heat dissipation. In addition, a relatively thick conductinggrid can be provided around the photocathode's active area (˜3×3μ²).

In this case, no in-plane current flows in the photocathode, so the onlysignificant potential difference across the highly resistivephotocathode material is between the conducting underlayer and theemitting surface. The electrical resistance between these layers isestimated as

${R = {R_{sq}\frac{l^{2}}{S}}},$

where R_(sq) is the sheet resistance (in ohms per square), l is thephotocathode thickness and S is the area of the photocathode. ForR_(sq)=10⁶-10⁷ Ω/sq, l=20-40 nm and S=9μ², the resistance R is in therange 50-2000Ω. For a current of 1 mA, this results in a voltage drop of0.05 V to 2 V across the photocathode. When currents of 10 μA flow insemitransparent photocathodes of conventional phototubes, thecorresponding in-plane voltages are at least of the same order ofmagnitude, since the resistance is of the order of MΩ. (Such currentsare allowed according to the tubes' specifications and do not destroythe photocathodes). Such voltages do not lead to manifestation of theelectrolytic decomposition. Consequently, the electrolytic decompositioneffect would be also avoided in an invented device. The Joule heatdissipated in the photocathode layer would amount to about 0.05 mW to 2mW.

The inventors have found that a required current density of 10,000 A/cm²can be obtained in a device with dimensions of 3×3 μm², meaning acurrent of ˜1 mA. It should be understood that the important parameteris a current density, and not the total current. This depends onapplying adequate illumination density. For example, blue laser diodescan be used to support advanced optical disk

drives. The reduced wavelength also yields more energetic electronswhich assist some critical device performance parameters. Photocathodematerials have been identified with yield around 20%, so generating theelectron flux is possible with practical photo-flux. Calculations showthat the reasonably high vacuum level needed to preserve thephotocathode, coupled with the small cavity volume of the device, meansthat positive ion bombardment can be discounted in our case. A thermalmodel has been created and analyzed showing that using a pulsedoperation at a duty ratio of 0.01 to 0.1 is immediately feasible, butgenerally continuous wave (CW) operation can be used as well. The issueof homogeneity has been considered showing that the constructionenvisaged would create a very homogeneous structure, so thatdifferential thermal stresses are unlikely to be present. Hencesecondary failure causes from this source are unlikely.

The inventors have found that the use of a photocathode layer on adiamond or sapphire substrate (good heat distributor) is preferred. Thisprovides heating at a few degrees Celsius for a 30×30μ² photocathodearea.

Thus, according to one broad aspect of the invention, there is providedan electric device operable with a THz-range frequency of the deviceoutput, the device comprising an electrodes' arrangement comprising aphotocathode electrode, an anode electrode and at least one gate wherethe photocathode is exposed to illumination of a predeterminedwavelength range thereby creating electrons' emission from thephotocathode and a photocurrent through the device; a voltage supplyunit for supplying an input electrical signal onto one of the electrodesthereby causing an output electrical signal readable on at least oneother electrode; and a signal transmitter/receiver circuit electricallyconnected to said at least one other electrode to be therefore operableby the output electric signal; the electrodes being spaced from oneanother a distance not exceeding a few microns, thereby allowing thedevice operation with a voltage supply of a few tens of volts or less tosaid at least one electrode and enable to obtain the electrical outputin the THz range of frequencies.

The device may or may not include an illumination source as itsconstructional part. The illumination is operable to illuminate thephotocathode with a light beam of certain fixed intensity and thepredetermined wavelength range selected in accordance with thephotocathode material for extracting electrons therefrom.

The electrodes' arrangement may be configured as a triode structureformed by the photocathode, the anode and the gate electrode locatedbetween the photocathode and anode. The gate electrode is preferablyspaced from each of the photocathode and anode planes a distance of lessthan 1 micron, e.g. about 0.1 micron gap between the gate and thephotocathode and about 0.3 microns gap between the gate and the anode.The gate may be of a 0.1 micron thickness and a 0.1 micron dimensionacross a space between the photocathode and the anode.

In some other embodiments of the invention, the electrodes' arrangementis configured as a tetrode structure formed by the photocathode, theanode, the gate and a screen grid electrodes. The latter is locatedbetween the gate and anode planes. The electrodes are spaced from oneanother a gap of less than 1 micron. For example, the gate is spacedfrom the photocathode a distance of 0.1 micron, and is spaced from thescreen grid a 0.3 microns gap, while the screen grid is spaced from theanode a distance of 0.3 microns. The thickness of the gate, as well asthat of the screen grid, may be a 0.1 micron.

The photocathode layer may be patterned to form an array of spaced-apartregions of the photocathode material and higher electrically conductivematerial within the spaces between the photocathode material regions.

According to another broad aspect of the invention, there is provided anelectric device operable with a THz-range frequency of the deviceoutput, the device comprising a diode structure formed by a photocathodeelectrode and an anode electrode; and an illumination source configuredand operable to generate a light beam in the form a superposition of twolight components of slightly different frequencies to thereby illuminatethe photocathode by an amplitude modulated wave and cause thecorresponding photocurrent to be received at the anode electrode tooperate signal transmitter/receiver circuit by an electric signalinduced by said photocurrent at the anode.

The illumination source may include two light emitters (preferably laserdiodes) generating the two light components of slightly differentfrequencies, respectively. The wavelengths may be for example 800 nm and801 nm.

Preferably, the photocathode and anode electrodes are spaced from eachother by a gap of a few microns or less.

A signal transmitter/receiver circuit is electrically connected to theanode. The diode thus operates to convert the THz light input into a THzelectrical output which in turn operates the signaltransmitter/receiver.

The photocathode and anode layers may be located on facing each otherspaced-apart surfaces of first and second substrates, respectively, andthe photocathode may be illuminated through an optically transparentsubstrate. The anode-supporting substrate is formed with an electricalconnector from the anode layer to an outer surface of the substrate forcoupling to a transmitting/receiving unit. The latter includes anantenna circuit, which may be printed on the substrate's surface.

According to yet another aspect of the invention, there is provided aTHz transmitter device comprising a diode structure comprising aphotocathode electrode and an anode electrode; and an illuminationsource configured and operable to generate a light beam in the form asuperposition of two light components of slightly different frequenciesto thereby illuminate the photocathode by an amplitude modulated waveand cause the corresponding photocurrent to be received at the anodeelectrode.

According to yet further aspect of the invention, there is provided aTHz transmitter device comprising an electrodes' arrangement comprisinga photocathode electrode, an anode electrode and at least one gate wherethe photocathode is exposed to illumination of a predeterminedwavelength range thereby creating electrons' emission from thephotocathode and a photocurrent through the device; a voltage supplyunit for supplying an input electrical signal onto one of the electrodesthereby causing an output electrical signal readable on at least oneother electrode; and a signal transmitter/receiver circuit electricallyconnected to said at least one other electrode to be therefore operableby the output electric signal; the electrodes being spaced from oneanother a distance not exceeding a few microns, thereby allowing thedevice operation with a voltage supply of a few tens of volts or less tosaid at least one electrode and enable to obtain the electrical outputin the THz range of frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 exemplifies an electronic system according to an example of thepresent invention;

FIG. 2A shows an amplitude modulated wave as a result of photomixing twoclose frequencies occurring in the system of FIG. 1;

FIG. 2B shows an effect of incorporating a Bragg Grating in a laserdiode;

FIG. 3 shows a specific example of the configuration of the system ofFIG. 1; and

FIGS. 4A and 4B exemplify triode and tetrode structure configured forthe purposes of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, there is exemplified an electronic system,generally at 10, according to the invented technique. The system 10includes a photoemission based electric device 12 (diode or triode), anillumination source 14, and a transmitter/receiver unit 16. The systemis typically associated with a control unit 18, which includes interalia an illumination controller 18A for controlling the

invention includes a voltage supply unit 18B for controlling electricalconditions of the electronic device 12, and may also include anappropriate control panel 18C.

In the present example, the electric device 12 is configured as a diodeincluding a photocathode 12A exposed to illumination from saidillumination source 14, and an anode 12B spaced from the photocathode apredetermined distance. The photocathode may be made of any suitablematerial, selected in accordance with the wavelength of light used, e.g.Bi-alkali, multi-alkali S₁₁, S₂₀, S₂₅, etc.

In the present example, the illumination source 14 is configured andoperable for producing an input light beam B_(in) presenting asuperposition of two light components λ₁ and λ₂ of a small frequencydifference, for example one being 800 nm and the other being 801 nm. Itshould be understood, although not specifically shown, that this can beimplemented using two light emitters (e.g. laser diodes) generating saidtwo light components, respectively, and an appropriate arrangement ofsuitable optical devices (such as mirrors, lenses, optical fibers, etc.)to direct the two beams towards propagation paths intersecting at thephotocathode, thereby obtaining a combined light beam in the form of asuperposition of these two light components.

Considering the light source (e.g. laser) radiation as a coherentsinusoid, if two nominally equal amplitude waves with a small frequencydifference are superimposed, the amplitude of the resulting wave, whosefrequency is equal to the sum of the source frequencies, is modulated ata “beat” frequency equal to the difference between the sourcefrequencies. This is the so-called “photomixing” technique. Morespecifically, the first and second light components have frequencies ω₁and ω₂ respectively, both being in the optical region of theelectromagnetic spectrum, but slightly differing from each other, thefrequency difference being in the THz range. The two light componentsoperate on the photocathode in heterodyne fashion so that the electronbeam is modulated at the beat frequency. Accordingly, the electric fieldincident on the photocathode is given by the sum of the electric fieldsof the respective light components, and the radiation power at thephotocathode is proportional to the square of the total electric field.The photocathode, while being incapable to respond by electrons'emission at frequencies ω₁, ω₂, or their sum, can respond at the beatfrequency. With (ω₁−ω₂)<<ω₁, ω₁≈ω₂, the photocathode acts as a lowfrequency filter and averages the incident power over time. If theincident power densities from the two lasers are equal, the incidentpower is 100% modulated at the beat frequency, and the photoemissionwill also be modulated at this frequency.

FIG. 2A shows an example of the dependence of the beat frequency in THzrange on the difference between the wavelengths of the light components,for the case when one of the wavelengths is 800 nm. For example, the twowavelengths may be λ₁=800 nm, λ₂=801 nm, and the difference (beatfrequency) is ˜0.5 THz.

FIG. 2B shows the amplitude modulated wave as a result of photomixingtwo close frequencies.

The light emitters used in the illumination source are preferably laserdiodes, because they can operate with relatively short wavelengths (e.g.about 800 nm) as required for efficient photocathode, and have a reducedline-width. Such a diode laser may incorporate a Bragg grating for thepurpose of narrowing and stabilizing the emission wavelength. Commerciallaser models exist today, with spectral linewidth of down to 10⁻⁵ nm.This is equivalent to a bandwidth of few MHz.

As indicated above, in some embodiments of the invention the diode isassociated with a voltage supply unit which maintains certain potentialdifference between the photocathode and the anode. It should beunderstood that alternatively, no predefined potential differencebetween them is needed: for example the photocathode and anode may beconnected to each other via an electrical connector (wire) while theflow of emitted electrons to the anode is initiate due to photoemissionof high kinetic energy electrons. In other words, high frequencyresponse can be achieved or supported by using high energy photons inthe photoemission process.

Due to the space charge effect, there is a change in the inter-electrodepotential resulting in a so-called “virtual cathode”, namely a lowpotential region near the cathode from which effective emission occurs.The effect of higher energy photons can be used to push this virtualphotocathode further away from the real photocathode.

Reference is made to FIG. 3 showing a specific but not limiting exampleof an electronic system generally at 20, according to some embodimentsof the invention. The same reference numbers are used for identifyingcomponents in the illustrations of FIGS. 1 and 3. The system 20 includesa diode structure 12 formed by spaced-apart photocathode 12A and anode12B, an illumination source (not shown here) which produces a THz inputlight modulation which is then converted by the diode device into THzelectrical radiation, and a transmitter unit 16 which transmits thisradiation. The photocathode 12A and anode 12B are supported by facingeach other surfaces of transparent substrates 13A and 13B, respectively,and are spaced from each other a few microns gap 15. The substrates aremaintained with a gap between them by means of a supporting spacer 17.As shown in the figure, the transmitter unit 16 includes an electricalconnector (conducting via) 19 extending across the anode relatedsubstrate 13B thereinside form the anode electrode 12B, and an antennacircuit (bow-tie antenna) 22 located (e.g. printed) on an externalsurface of the anode substrate 13B, being electrically coupled to theconducting via 19. The photocathode and anode may be kept at,respectively zero and 20V voltages. The system 20 thereby presents a THzantenna arrangement operable at low voltages.

In the above-described examples, a THz transmitting system is achievedusing a diode structure formed by photocathode and anode spaced fromeach other a small distance, and heterodyne fashion illumination of thephotocathode. As will be described below, such a THz transmitter can beobtained using a triode or tetrode structure.

In the devices of the invention, the photocathode is adjusted forhigh-density current (10³ A/cm²), which can be achieved due to theoptimal heat distribution as described above, namely using a highly heatdistributing substrate (diamond or sapphire) and the use of highlyelectrically conductive material over which the photocathode isevaporated—such as thin transparent metal (e.g. ˜30 Å of Chrome). Toachieve even higher electrical conductivity of the photocathode layer, athicker conductive non transparent grid may be placed beneath thephotocathode layer, dividing its area to sub areas, and thus reducingits overall electrical resistivity.

Preferably, the cathode-anode spacing (gap) is very low, e.g. a fewmicrons. This enables reaching high current densities at low voltages,without space charge limiting the current. The light source is alsoappropriately selected to provide the required density of photon flux.For example for the current density of 10⁴ A/cm²) photocurrent of about0.1 A, the light density of 10⁵ W/cm² is needed.

The temporal response of a photocathode, while being generally limitedby electron diffusion, can practically be 50-200 femto-seconds, whichsupports photo-current at frequencies of up to at least 3 THz. The lowerobtainable frequency is limited by the laser diode line-width, or moreprecisely by the convolution of the two laser diodes' line-widths. Forsubstantially identical laser diodes whose spectral lines are modeled byGaussians, a spectral width at THz proportional to the single laserdiode line-width can be obtained.

The efficiency of THz CW production is much higher when using aphotocathode, than that using a low-temperature-grown-GaAs photodiode.In the latter technique, only a few microwatts are transmitted, while inthe photocathode based diode exposed to similar light sources (diodelasers) about 1 mW of THz radiation can be transmitted.

Reference is made to FIGS. 4A and 4B illustrating examples of triode andtetrode structures 40A and 40B suitable to be used as a photoemissionbased electric device in a THz transmitter/receiver system of thepresent invention. It should be understood, although not specificallyshown here, that such a system includes an illumination source forgenerating a light beam of an appropriate wavelength range and powerprofile for emitting electrons from the photocathode arranged to beexposed to this illumination, and a control unit including anillumination control utility and a voltage supply control utility. Thephotocathode is exposed to certain fixed illumination, an inputelectrical signal at THz region is supplied to one of the electrodes(e.g. the gate), and the device operation (photocurrent) amplifies theinput signal, and the amplified output is read at another electrode(e.g. the anode). The examples described below show how the electrodes'arrangement (dimensions and distances between the electrodes) of thetriode/tetrode electric device allows for reaching THz rangefrequencies. These devices are configured with special construction ofthe photocathode and very small dimensions of electrodes' and spacesbetween them and is operable with suitably small voltages to enable highcurrent density and high frequency (up to THz range) electrical output.

As shown, in the triode 40A, the photocathode is spaced from the 0.1 μmthick gate a distance of 0.1 μm, and the gate further is spaced from theanode a distance of 0.3 μm. In the tetrode 40B, a screen grid is placedbetween the gate and anode planes being spaced from each of them the 0.3μm distance. The dimension of the gate and screen grids' elements acrossthe gap between the electrodes is 0.1 μm, leaving a lateral space of 0.3μm for the electrons propagation.

The inventors have shown that the construction of a THztransmitter/receiver system utilizing a photocathode is feasible, andcan provide a required current density. The use of a small gap betweenthe electrodes of the electric device used in this system implies adesirable space-charge limit at ˜10,000 A/cm². The entire device may beof sub-micron dimensions, such that the transit time of electronsthrough the device is much shorter than the highest frequency of theprocessed signal.

1. An electric device operable with a THz-range frequency of the deviceoutput, the device comprising an electrodes' arrangement comprising atleast a photocathode electrode and an associated anode electrode wherethe photocathode is exposed to illumination of a predeterminedwavelength range thereby creating electrons' emission from thephotocathode and a photocurrent through the device; a voltage supplyunit for supplying an input electrical signal onto one of the electrodesof said electrodes' arrangement, an output electrical signal of aTHz-range frequency being readable on one or more of the electrodes ofsaid electrodes' arrangement; and a signal transmitter/receiver circuitelectrically connected to the electrode at which the device output isread to be therefore operable by the output electric signal.
 2. Thedevice of claim 1, comprising an illumination source operable toilluminate the photocathode with a light beam of certain fixed intensityand said predetermined wavelength range selected in accordance with thephotocathode material for extracting electrons therefrom.
 3. The deviceof claim 1, wherein the electrodes' arrangement is configured as atriode structure comprising the photocathode, the anode and the gateelectrodes, the gate being located in a plane between the photocathodeand anode planes.
 4. The device of claim 3, wherein the gate electrodeis located in a plane spaced from each of the photocathode and anodeplanes a gap of less than 1 micron.
 5. The device of claim 4, whereinthe gap between the photocathode and anode is about 0.1 micron and 0.3microns, respectively.
 6. The device of claim 5, wherein said gate has athickness of 0.1. micron and a dimension across a space between thephotocathode and the anode of about 0.1 micron.
 7. The device of claim3, wherein the electrodes' arrangement is configured as a tetrodestructure comprising the photocathode, the anode, the gate and a screengrid electrodes, the screen grid being located between the gate and theanode planes.
 8. The device of claim 7, wherein the electrodes arespaced from one another a gap of less than 1 micron.
 9. The device ofclaim 8, wherein the gap between the gate electrode and the photocathodeis 0.1 micron, the gap between the gate and screen grid is 0.3 microns,and the gap between the screen grid and the anode is 0.3 microns. 10.The device of claim 9, wherein the thickness of the gate and of thescreen grid is 0.1 micron.
 11. The device of claim 1, wherein thephotocathode layer is patterned to form an array of spaced-apart regionsof the photocathode material and higher electrically conductive materialwithin the spaces between the photocathode material regions.
 12. Anelectric device operable with a THz-range frequency of the deviceoutput, the device comprising a diode structure comprising aphotocathode electrode and an anode electrode; and an illuminationsource configured and operable to generate a light beam in the form asuperposition of two light components of slightly different frequenciesto thereby illuminate the photocathode by an amplitude modulated waveand cause the corresponding photocurrent to be received at the anodeelectrode to operate a signal transmitter/receiver circuit by anelectric signal induced by said photocurrent at the anode.
 13. Thedevice of claim 12, wherein the illumination source comprises two lightemitters generating said two light components of slightly differentfrequencies, respectively.
 14. The device of claim 13, wherein one ofsaid two light components is of 800 nm wavelengths.
 15. The device ofclaim 13, wherein said two light emitters are laser diodes.
 16. Thedevice of claim 13, wherein one of said two light components is of 800nm wavelengths and the other of 801 nm.
 17. The device of claim 12,wherein the photocathode and anode electrodes are spaced from each otherby a gap of up to a few microns.
 18. The device of claim 12, comprisinga signal transmitter/receiver unit electrically connected to the anode.19. The device of claim 12, wherein the photocathode and anode layersare located on facing each other spaced-apart surfaces of first andsecond substrates, respectively.
 20. The device of claim 19, wherein thefirst substrate is substantially transparent for said light beam. 21.The device of claim 19, wherein the second substrate is formed with anelectrical connector from the anode layer to an outer surface of saidsecond substrate for coupling to a transmitting/receiving unit.
 22. Thedevice of claim 18, wherein the transmitter/receiver unit comprises anantenna circuit.
 23. The device of claim 21, wherein thetransmitter/receiver unit comprises an antenna circuit printed on theouter surface of said second substrate.
 24. A THz transmitter devicecomprising a diode structure comprising a photocathode electrode and ananode electrode; and an illumination source configured and operable togenerate a light beam in the form a superposition of two lightcomponents of slightly different frequencies to thereby illuminate thephotocathode by an amplitude modulated wave and cause the correspondingphotocurrent to be received at the anode electrode.
 25. A THztransmitter device comprising an electrodes' arrangement comprising atleast a photocathode electrode and an associated anode electrode wherethe photocathode is exposed to illumination of a predeterminedwavelength range thereby creating electrons' emission from thephotocathode and a photocurrent through the device; a voltage supplyunit for supplying an input electrical signal onto one of theelectrodes, an output electrical signal readable on one or moreelectrodes of the electrodes' arrangement being of a THz-frequencyrange; and a signal transmitter/receiver circuit electrically connectedto the electrode at which the device output is readable to be thereforeoperable by the output electric signal.
 26. The device of claim 1,wherein said electrodes' arrangement comprises at least one gateelectrode located in a plane between the photocathode and anode planes.27. The device of claim 1, wherein the electrodes are spaced from oneanother a distance not exceeding a few microns, thereby allowing thedevice operation with a voltage supply of a few tens of volts or less tosaid at least one electrode.